LumaSense Technologies MCS640 Manual - Manual (Page 19)

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Section 4 Principles of Thermal Imaging
Absorptivity equals emissivity, thus emissivity can be described by reflectivity and
transmissivity.
a + t + r = 1
In order to obtain the true temperature of an object, it is necessary to obtain the
emissivity correctly. Therefore, the emissivity of the object has to be measured by using a
blackbody-type source which is closest to an ideal blackbody as possible. The blackbody-
type source can be designed to meet the conditions pointed out by Kirchoff where “the
radiation within an isothermal enclosure is blackbody radiation.”
As a blackbody-type source for a measurement must radiate outside of the enclosed
surface, a small hole is cut through the wall of the enclosure small enough not to disturb
the blackbody condition. The radiation leaving this hole should closely approximate
that of a blackbody. When the diameter of the hole is as 2r and the depth is as L, if L/r
is equal or more than 6, it is used as a blackbody-type source for practical use. The
following figure shows an example of a blackbody-type source based on blackbody
conditions.
4.5 Determining Emissivity
Emissivity is the ratio of energy radiated from an object to the exterior and energy
radiated from a blackbody. The emissivity varies with the surface condition of the object
and also with temperature and wavelength. If this value is not accurate, then the true
temperature cannot be measured. In other words, a variation or change in emissivity will
cause a change in the indications on a thermal imager.
To approach the true temperature therefore,
The emissivity must approximate 1.0 ( The measured object must be
nearly a blackbody.)
The emissivity must be corrected. ( The emissivity of the measured
object must be internally corrected to 1 by the thermal imager.)